Mercury concentrations in the fish are rising by 3.8 percent or more a year, they found after analyzing data from 1971, 1998 and 2008.

"The take-home message is that mercury in tuna appears to be increasing in lockstep with data and model predictions for mercury concentrations in water in the North Pacific," said Paul Drevnick, an assistant research scientist at the University of Michigan School of Natural Resources and Environment.

"This study confirms that mercury levels in open ocean fish are responsive to mercury emissions," Drevnick added in a university news release.

Yellowfin tuna, sold as ahi, is widely used in raw fish dishes -- especially sashimi -- and for grilling. This type of tuna is listed as a "high mercury" species by the U.S. Natural Resources Defense Council.

Mercury is a potent toxin, and high concentrations in fish pose a health risk to people who eat them.

The main source of mercury in the open ocean is fallout from air pollution, especially from coal-fired power plants and artisanal gold mining, according to the authors of the study presented Feb. 2 in the journal Environmental Toxicology and Chemistry.

"Mercury levels are increasing globally in ocean water, and our study is the first to show a consequent increase in mercury in an open-water fish," Drevnick said. "More stringent policies are needed to reduce releases of mercury into the atmosphere. If current deposition rates are maintained, North Pacific waters will double in mercury by 2050."